In many optical image transmission systems, it is desired that the transmitted image information have a gray scale characteristic, that is, that the transmitted information concerning a given localized area be characterized by multi-level information capability rather than merely a binary (bright vs. dark) level characteristic. One system for transmitting such multi-level or gray scale information involves the technique of "dither" coding. In this technique, a plurality of typically sixteen (16) different possible optical intensity threshold levels are pre-assigned in a given spatial pattern, and the brightness of each image point is compared with a preselected one of these sixteen levels. The outcome of each comparison is represented by a binary digital signal, "1" or "0". Thus, if a given first localized area is brighter than a given second localized area, more of the image points within the first area will have greater brightness ("positive" response) than the pre-assigned dither threshold levels, and thus there will be more image points within the first area than in the second area which result in "positive" responses with respect to the dither coded pattern. Accordingly, the average optical intensity of a given area corresponds to the number of "positive" responses, each such response represented by the binary digital "1", as opposed to "negative" responses (lesser brightness than dither threshold levels), each such negative response represented by the binary digital "0". Upon transmission and display of the corresponding two-dimensional pattern of all these 1's and 0's by means of bi-level display cells, the human eye observer obtains the desired gray scale impression of local relative intensity, in accordance with the relative number of cells that are "positive" vs. "negative".
Various two-dimensional spatial patterns for the pre-assigned dither coding levels can be pre-assigned, in order to minimize various systematic or other errors resulting from the point sampling inherent in the dither technique. Moreover, as disclosed in U.S. Pat. No. 3,953,668, issued to C. N. Judice on Apr. 27, 1976, the flicker which results in dither image displays in an interlaced optical field format can be eliminated by localized intensity averaging of a cluster of cells. However, regardless of the selected dither coding pattern or "matrix", there remains another source of error, namely, nonuniform background illumination.
As shown in FIG. 1, the background illumination I extending from x.sub.1 to x.sub.2 over an object 11, such as a line of a page of print to be imaged by an optical imaging system, various with location x. Accordingly, if the imaging system is to be characterized by a dither coded gray scale capability, a serious error is introduced if the intensity of each image point is compared with a predetermined fixed dither coded level of illumination, that is, if the dither level is not corrected for nonuniform background illumination.